Fox?

We have had free-range chickens for three years, and never lost one to a fox. Our dogs scare them away. Out dog wont go into the cow fencing, and the fox found that out. He only takes white chickens, even though all the chickens sttay near eath other, the fox only takes one, and leaves no feathers. He never takes roosters. We have seen the fox, and tryed to shoot him, but is this an odd fox? We have no white chickens left and he has not come for months. Why only white? no feathers? only one at a time?